Internal combustion engines on the basis of gasoline engines are generally operated with fuel from hydrocarbons, from fossil fuels based on refined crude oil. Ethanol produced from renewable resources (plants) or another kind of alcohol is increasingly being added in various mixing ratios to the fuel. In the USA and Europe a mixture of 70-85% ethanol and 15-30% gasoline is often distributed under the trade name E85. The internal combustion engines are designed in such a way that they can be operated with pure gasoline as well as with mixtures up to E85. This is denoted as a “flex-fuel operation”. The operating parameters in the flex-fuel operations have to be adapted to the respectively existing fuel mixture for an efficient operation with only a small discharge of toxic emissions, while at the same time a high degree of engine performance is maintained. A stoichiometric fuel-air mixture ratio is, for example, present at 14.7 volumetric parts of air per part of gasoline; however, when using ethanol, a proportion of air of 9 volumetric parts must be set. Small and/or slow changes in the ethanol content can be detected and taken into account by the engine management system of the internal combustion engine by means of a lambda probe and/or a knock sensor. Rapid changes with a significant deviation in the composition of the fuel mixture can also occur, for example, after filling the tank (fueling). If the internal combustion engine were operated with 100% gasoline and filled with E85 when the tank was close to empty, problems in starting and disturbances in the combustion can arise, which can also increase the harmful exhaust gas emissions. According to the state of the art, such rapid changes in the composition of the fuel can be detected using an ethanol sensor. This component, however, increases the cost of the internal combustion engine.
A method is known from the German patent DE 4117440 C2 for the adaptive adjustment of a fuel-air mixture to take into account fuel characteristics in the operation of an internal combustion engine, which has a lambda controller, which transmits a control factor RF, and which has an adaptation integrator, which transmits an adaptation factor AF with a variable adaptation speed. Beside the control factor RF, said adaptation factor AF influences the adjustment of the fuel-air mixture. Provision is thereby made for a test to determine if the lambda control deviation amplitude exceeds an initial threshold value. If this is the case, the adaptation speed is set to an increased value until a specified condition is filled, whereby the controller switches back to a base adaptation speed.
The method makes it possible to operate internal combustion engines, which can be operated with different fuels, trouble-free. Thus, the injection time must, for example, be lengthened by more than 20% in order to maintain the same lambda values in the exhaust gas, when a change is made from pure gasoline to a fuel mixture with 85% ethanol and 15% gasoline. According to the method described in the text of the German patent DE 4117440 C2, an appropriate adaptation intervention is performed for this purpose. Because a very large correction in the injection times and consequently in the adaptation intervention must be undertaken when a change in fuel occurs in comparison to the compensation for factors of wear and manufacture, the adaptation speed is significantly increased in the proposed method when a change in fuel is detected.
The fuel mixture ratio can be determined on the basis of the adjusted adaptation value. Despite the increased adaptation speed, the method requires a sufficiently long settling time. If a significant change in the fuel mixture ratio is caused by a filling of the tank (fueling), this can lead to problems in starting and misfires, which successively lead to an increase in exhaust gas emissions.
It is the task of the invention to provide a method, which allows for a fast and cost effective detection of the composition of a fuel mixture from fuels with different boiling points and/or enthalpies of evaporation.